Specific aims: 1) To determine the long term effect of a strategy of chemical or electrical cardioversion (and long term pharmacologic suppression) of atrial fibrillation on left ventricular function, exercise tolerance, and intrinsic cardiac contractility. 2) If long term improvement in cardiac function occurs, to determine the time course and mechanism of that improvement. Hypotheses to be tested: 1) Intrinsic cardiac contractility is impaired in patients with chronic atrial fibrillation. 2) The mechanism of that impairment is: A) Persistent tachycardia. B) Abnormal diastolic filling leading to diastolic dysfunction. 3) Long term ventricular function can be improved if chronic atrial fibrillation is successfully suppressed.